Re: [patch 5/4] mm: document is_page_cache_freeable()

["Li, Ming Chun" <macli@brc.ubc.ca>]

On Thu, 23 Jul 2009, Johannes Weiner wrote:

> On Wed, Jul 22, 2009 at 02:55:12PM -0700, Li, Ming Chun wrote:
> > On Wed, 22 Jul 2009, Christoph Lameter wrote:
> > 
> > > 
> > > >  static inline int is_page_cache_freeable(struct page *page)
> > > >  {
> > > > +	/*
> > > > +	 * A freeable page cache page is referenced only by the caller
> > > > +	 * that isolated the page, the page cache itself and
> > > 
> > > The page cache "itself"? This is the radix tree reference right?
> > > 
> > 
> > I think you are right. I had trouble understanding this function, So I 
> > looked into it and found out the call path:
> > 
> >  add_to_page_cache_locked 
> >    -> page_cache_get
> >     -> atomic_inc(&page->_count) 
> > 
> > Please correct me if I am wrong.
> 
> This is correct.  But this is the purpose of reference counters - you
> increase it when you reference the object so that it doesn't get freed
> under you.
> 
> That's why everybody holding a reference to the page must have its
> usage counter increased.  And this includes the page/swap cache, the
> LRU lists, the page tables etc.
> 
> And I think in that context my comment should be obvious.  Do you need
> to know that the page cache is actually managed with radix trees at
> this point?
> 
> You need to know that the page cache is something holding a reference
> to the page so you can meet the requirements that are written above
> remove_from_page_cache() - which you are about to call.
> 
> I added the comment to document that magic `compare with 2' in there.
> If more is needed, I am glad to help - but right now I don't really
> think I know what the issue is with this patch?
> 
> 
No issue at all for me :), I should clarify that I had problem 
understanding it before you post your comment patch, Your comment acutally 
helped me to undertand it better and thanks for the comments. I am just learning from you  by 
lurking in mailing list and reading the code to understand what you comments.


Vincent Li
Biomedical Research Center
University of British Columbia

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